1. Field of the Invention
This invention relates generally to thermography and, more particularly, to using thermography to identify material characteristic flaws in articles made of composite materials.
2. Description of Related Art
Thermography has been used to locate material characteristic flaws in articles including articles made of composite materials. Composite materials include both polymers and ceramics. Thermography has been used to show flaws appearing in composites via their characteristic time-temperature contrast signatures obtained with infrared thermography. Thermography has been used to locate the flaws that alter the flow of heat but did not identify what types of material characteristic flaws were found. Material characteristic flaws in composites typically include porosity and delaminations as contrasted to size, location, and depth of flaws.
Thermography is a non-destructive evaluation (NDE) technique in which heat is applied to an object and a resulting temperature distribution on a surface of the object is measured over time with an infrared camera. Information about flaws in the object is obtained from the recorded infrared time sequence. Images are digitized into picture elements, or pixels, each representing a small unit area on the object's surface. A temperature/time signal is processed and evaluated per pixel and in patterns of pixels.
One known contemporary application of transient thermography, that provides the ability to determine the size and “relative” location (depth) of flaws within solid non-metal composites, is disclosed in U.S. Pat. No. 5,711,603 to Ringermacher et al., entitled “Nondestructive Testing: Transient Depth Thermography”; and is incorporated herein by reference. The method disclosed therein involves heating the surface of an object of interest and recording the temperature changes over time of very small regions or “resolution elements” on the surface of the object. The method provides for determining a size and a value indicative of a “relative” depth of a flaw (i.e., relative to other flaws within the object) based upon a careful analysis of the temperature changes occurring at each resolution element or pixel over the surface of the object.
U.S. Pat. No. 6,367,968, entitled “Thermal Resonance Imaging Method” to Ringermacher et al., discloses an infrared (IR) transient thermography system for producing a sequence of image frames acquired from an IR sensitive focal-plane array camera. U.S. Pat. No. 6,367,969, entitled “Synthetic Reference Thermal Imaging Method” to Ringermacher et al., discloses an infrared (IR) transient thermography system for producing a sequence of image frames acquired from an IR sensitive focal-plane array camera. Each frame is made up of an array of pixels and has assigned a frame-number that corresponds to elapsed time. Temperature-versus-time (T-t) data corresponding to each pixel is developed from stacks of sequential image-frames. Also disclosed is a method of analyzing the stacks of thermal data image frames to determine the thickness of an object and produce a color-keyed or gray-scale coded thickness map. U.S. Pat. No. 6,367,969, entitled “Synthetic Reference Thermal Imaging Method” to Ringermacher et al., discloses an infrared (IR) transient thermography system in which a sequence of image frames is acquired from an IR sensitive focal-plane array camera. Each sequentially acquired image frame is made up of an array of pixels and has assigned a frame number that corresponds to elapsed time. A method of analyzing thermal imaging data-frames is presented wherein a synthetically generated temperature-time reference curve is used to determine pixel contrast-versus-time data. U.S. Pat. No. 6,367,968 and U.S. Pat. No. 6,367,969 are both assigned to the General Electric Company, the assignee of this patent, and are incorporated herein by reference.
A surface of an object is heated to a particular temperature in a sufficiently short period. Depending on the thickness and material characteristics of the object under test, a quartz lamp or a high intensity flash-lamp is conventionally used to generate a heat pulse of the proper magnitude and duration. Any means capable of quickly heating the surface to a temperature sufficient to permit thermographic monitoring—such as, for example, pulsed laser light may be used. Once the surface of the object is heated, a graphic record of thermal changes over the surface is acquired and analyzed as the object cools down.
An infrared (IR) video camera is used to record and store successive thermal images (frames) of an object surface after heating it. Each video image is composed of a fixed number of pixels. In this context, a pixel is a small picture element in an image array or frame that corresponds to a rectangular area, called a “resolution element”, on the surface of the object being imaged. Since, the temperature at each resolution element is directly related to the intensity of the corresponding pixel, temperature changes at each resolution element on the object surface can be analyzed in terms of changes in pixel contrast. The stored IR video images are used to determine the contrast of each pixel in an image frame by subtracting the mean pixel intensity for a particular image frame, representing a known point in time, from the individual pixel intensity at that same point in time.
Several systems and methods, including ones in the patents cited above, disclose how to use this data to identify and determine depths of flaws have been disclosed. What they do not disclose is how to characterize or identify what material characteristic type of flaws were found such as delaminations and porosity. It is desirable to have a nondestructive testing method to find flaws in composite articles and identify what type of material characteristic flaws were found. It is particularly desirable to have a nondestructive testing method to find material characteristic flaws and identify or discriminate between porosity and delaminations.